International Business Machine Corporation (IBM) mainframes provide online transaction processing subsystems, such as Information Management System (IMS®) and Customer Information Control System (CICS®) (IMS and CICS are registered trademarks of IBM), that enable high-speed transaction processing of data stored in file structures. Each processing subsystem has file structures unique to their environment. For example, IMS transactions access data stored in IMS databases and CICS transactions access data stored in VSAM data sets. For various reasons, the majority of data is stored in IMS databases. Generally, applications and transactions running outside of the subsystems could not access the data stored within the subsystem.
To allow outside access to subsystems, IBM first introduced an interface allowing CICS transactions to have high-speed access to IMS databases. The interface allows CICS to access IMS databases while the databases are available for online IMS transaction processing. This interface is known as the IMS Database Resource Adapter or DRA. DRA requires CICS systems to connect to a separate IMS system running on the same system or logical partition (LPAR) as the CICS system. Each LPAR is, in practice, equivalent to a separate mainframe. Thus, for an instance of CICS to have access to an IMS database, IMS must be running on the same system as the instance of CICS. This results in multiple installations of IMS systems at a customer site, for example on multiple LPARs within a sysplex. The costs that result from the multiple installations can be prohibitive, leaving some customers without the ability to integrate CICS transactions with IMS databases. While IBM has introduced other interfaces that allow programs to have access to remote IMS databases, such interfaces require a change to the business application source code. Many legacy business application programs still use the DL/I interface.